1. Field of the Invention
The present invention relates to an implantable medical device for tissue stimulation such as heart pacemakers or defibrillators, and more particularly, to cardiac stimulating devices of the type having circuitry for removing noise in sensed electrical signals.
2. Description of the Prior Art
A pacemaker provides electrical stimulation pulses to the right atrium and/or the right ventricle of the heart in order to stimulate the muscle tissue to cause a contraction.
Demand pacemakers monitor the heart, through the same electrical leads through which the stimulation pulses are provided, in order to sense the occurrence of a P wave and /or R wave (xe2x80x9cP/R wavexe2x80x9d). If a P/R wave is sensed, then there is no need to deliver a stimulation pulse. In such an instance (when a P/R wave is sensed), the delivery of the stimulation pulse in a demand pacemaker is inhibited, thereby conserving the limited power of the pacemaker""s battery, and further preventing irregular rhythms (contractions) of the heart muscle tissue that might otherwise result. Thus, a demand pacemaker provides stimulation pulses to the right atrium and/or right ventricle on demand, i.e. only when needed.
In dual chamber pacemakers there is not only an inhibiting function but also a trigged function. The sensing of a P wave inhibits the atrial stimulation. It furthermore triggers an AV interval, which stimulates the ventricle, if a ventricle activity has not been sensed before the AV interval runs out. If a ventricle activity is sensed, the ventricle stimulation is inhibited.
Similarly, automatic defibrillators provide a high energy stimulation pulse to cardiac tissue in an attempt to start contractions in a heart that has stopped. If the heart responds to such high energy defibrillation pulses and starts beating on its own, the need for defibrillation also operates in a demand mode, providing defibrillation pulses only when needed.
The ability of e.g. a demand pacemaker, dual chamber pacemaker or automatic defibrillator to properly perform its function of providing stimulation pulses on demand is critically dependent upon its ability to detect P/R waves. Unfortunately, many electrical signals may be present in a typical ECG signal (that signal sensed through the pacemaker or defibrillator leads) that do not represent valid P/R waves. Such signals are referred to as noise. Body movements may give rise to noise.
The stimulation of heart tissue and the detection of P/R waves is either provided by a unipolar lead or a bipolar lead. Unipolar stimulation and sensing is achieved between the tip electrode of the lead and the indifferent electrode of the tissue stimulating device, and bipolar stimulation and sensing is achieved between the tip electrode of the lead and the ring electrode of the tissue stimulating device. A bipolar lead may provide unipolar stimulation or sensing.
Thus, compared to a unipolar lead, a bipolar lead needs two insulated connections and wires for proper operation. The bipolar lead is therefore more sensitive to fatigue breakage, short-circuits and insulation defects. The ring electrode also renders the bipolar lead less flexible in the area of the ring electrode, and since the ring electrode is normally placed a few centimeters from the tip electrode where there is maximum bending of the lead, this results in fatigue breakage around the area of the ring electrode. Consequently, unipolar leads are for mechanical and quality reasons superior to bipolar leads.
However, in the bipolar system both the tip electrode and the ring electrode are placed inside the bean, and since the distance between the electrodes is small compared to the distance to muscles associated with body movements, the muscle signals affect the electrodes in much the same manner and consequently these muscle signals cancel each other. In the unipolar system the indifferent electrode is in close contact with muscles relating to body movements. The indifferent electrode is very likely to pick up signals from e.g. arm movements, and these signals can reach amplitudes far higher than the amplitude registered by the tip electrode as heart signal. Consequently, bipolar leads have a superior noise suppression and in particular of muscular interference.
One technique that can be used to reduce noise in a pacemaker is disclosed in U.S. Pat. No. 5,010,887 which relates to a noise discrimination circuit in implantable pacemakers. The noise discrimination circuit monitors the ECG signal to determine both the amplitude and duration of any signal pulses appearing thereon. If the amplitude of a given ECG signal pulse exceeds a prescribed threshold level for a prescribed duration, the pulse is considered to be a valid ECG signal.
U.S. Pat. No. 5,522,857 discloses another technique whereby a pacemaker comprises means for detecting the presence of noise by using two different escape intervals.
European Application 0 713 714 relates to an implantable medical device which comprises a correlator for producing a correlation function indicating the level of an EMIL (electromagnetic interference) component of an input heart signal that includes a heart signal component and an EMI component. The correlation function is monitored relative to a predetermined threshold to allow the device to operate in a normal manner as long as the intensity or level of the EMI component is such that it does not affect or interfere with the device operation. One or more remedial measures may be selected in the event the level of the EMI component exceeds a predetermined threshold.
It is an object of the present invention to provide a cardiac stimulator having a more reliable arrangement for reducing electrical noise signals originating from the human body without having to monitor the duration and/or amplitude of the sensed electrical signal.
The above object is achieved in accordance with the principles of the present invention in a medical device adapted for implantation in a human body for stimulating heart tissue having a housing which includes an indifferent electrode, an electrode lead connectable to the housing which has a tip electrode for sensing electrical signals originating within the heart tissue, and a noise sensing electrode connected to the housing for sensing electrical noise signals from the human body which originate outside of the heart tissue, the noise sensing electrode being electrically insulated from the indifferent electrode.
An advantage of providing a noise sensing electrode located outside the heart for sensing noise signals originating outside the heart is that there is no need for determining what is to be considered as noise, since everything sensed by the noise sensing electrode is considered to be noise.